Method for fabricating chirped fiber bragg gratings

ABSTRACT

A chirped Bragg grating is fabricated in an optical fiber by exposing the fiber to a coherent beam of light through a parallel phase mask having a series of progressively chirped segments produced on a lithography tool. The chirped phase mask is fabricated by exposing a photoresist-coated substrate to an image writing element such as an electron beam or a laser according to a set of parameters provided to the lithography tool. The parameters include a basic grating pattern for each segment, a value that defines the expansion or contraction of the grating pattern and an axis location to which the grating pattern is to be written to the substrate. By selecting machine commands that implement these parameters with a minimum throughput overhead, the mask can be produced in a reduced time, and therefore with increased accuracy.

FIELD OF THE INVENTION

This invention relates to optical fiber Bragg gratings and, inparticular to an improved method for writing masks for use infabricating in-fiber chirped Bragg gratings.

BACKGROUND OF THE INVENTION

Optical fibers are essential components in modern telecommunicationssystems. Comprised of thin strands of glass, optical fibers enable thetransmission of light, or optical signals, over long distances withlittle loss. An optical fiber typically has a core of glass having aspecific index of refraction surrounded by a glass cladding having alower index of refraction. Thus, light entering the fiber is retainedwithin the core by internal reflection.

In applications wherein a single optical fiber carries signals on morethan one wavelength, such as in wavelength division multiplexing (WDM),fiber Bragg gratings are used for controlling the specific wavelengthsof light within the fiber. Fiber Bragg gratings also have otherapplications such as in fiber lasers. A typical Bragg grating comprisesa length of optical fiber having periodic modulations in the index ofrefraction in its core, spaced equally along the length of the grating.

Several methods have been developed to fabricate fiber Bragg gratings.For example, the holographic, or interferometer method uses theinterference patterns created at the intersection of two coherent lightbeams to induce index modulations directly in the optical fiber. Asecond fabrication method involves the use of a phase mask positionedclose and parallel to an optical fiber on which the grating is to beformed. For example, by placing the phase mask between a coherent lightsource, such as a laser, and an optical fiber, the diffraction caused bythe mask replicates the function of an interferometer, creating aplurality of divergent light beams that interfere with each other in apredictable pattern, resulting in periodic alterations in the refractiveindex of the exposed core of the optical fiber. Typically, fiber Bragggrating fabrication using a phase mask requires stripping away thecladding before exposure of the core, although gratings can be printedonto an unstripped fiber having a cladding that is transparent to thewavelength of light passing through the phase mask.

Because of their ability to selectively reflect specific wavelengths ina narrow bandwidth, while allowing the remaining wavelengths to passessentially unimpeded, Bragg gratings are used as filters, stabilizers,dispersion compensators and for other applications in fiber opticsystems. It is desirable, however, under certain circumstances tobroaden the range of wavelengths affected by a Bragg grating. Toaccomplish this, a technique known as chirping is applied wherein thespacing between the periodic modulations in the refractive index (pitch)of an ordinary Bragg grating is gradually increased or decreased alongthe length of the grating. Thus a chirped fiber Bragg grating has awider active bandwidth and a wavelength-dependent time delay because ithas a wider range of spacings.

Although the characteristics of chirped fiber Bragg gratings aredesirable, fabrication of such gratings has proven difficult andtime-consuming. Particularly, the fabrication of chirped phase masks hasbeen challenging. For example, a typical chirped Bragg grating phasemask may have an array of between 100 to 200 grating segments, eachbetween 0.5 and 1.0 mm in length and with a pitch change betweensegments measured on the picometer scale. The pitch of each successivesegment varies continuously from, and must be “stitched” or placedprecisely relative to, the preceding segment. Conventional lithographytools such as an electron beam (e-beam) tool (for example, the MEBES IIIor MEBES 4500, both manufactured by Applied Materials, Inc. of SantaClara, Calif.) have been unable to achieve the accuracy necessary toproduce a phase mask for a chirped Bragg grating in a time period thatmakes them competitive.

A lithography tool uses an image writing element such as a laser beam oran electron beam to print an image, such as that of a phase mask onto asubstrate. Thus exposed, the substrate can be processed such that agrating pattern comprising, for example, an array of alternating linesand spaces is etched into the substrate. On a MEBES tool in particular,phase mask fabrication has been attempted using a “scale factor”approach and is therefore particularly complex. Specifically, a basicunscaled segment, or grating pattern, is established having apredetermined address unit defining its size. The grating pattern isrescaled as needed by applying scale factors to the address unit. Scalefactors are dimensionless values that are applied by the MEBES tool tothe address unit to achieve the desired reduction or magnification ofthe grating pattern. When applied across an entire mask, a specificchirp, or rate of change in the grating period of the finished phasemask is the result.

FIG. 1 is a block diagram of the steps in fabricating a chirped fiberBragg grating from a phase mask produced applying the scale factormethod on lithography tool such as a MEBES tool which uses an electronbeam as an image writing element. The first step, shown as 10, is toprovide a photoresist coated substrate. As is common in the art, thesubstrate is often approximately 152×152×6.35 mm and is typically formedof amorphous quartz, due to its ability to transmit ultraviolet light,or of some other substantially transparent material. One of the majorsurfaces of the substrate is typically coated with 3000 to 5000angstroms of photoresist material such as PBS or ZEP7000 over 1000angstroms of a Cr/Cr oxide layer.

The second step, shown as 20 in FIG. 1, is to provide a grating patternand the necessary scale factor and address unit values to the MEBEStool. The scale factor value to be applied to the grating pattern foreach grating segment to be written onto the mask is established duringthe design and is known prior to the fabrication of the mask.

TABLE 1 Scale Factor jobdeck example CHIP 1, (1, PHASEDE-MO-TK, AD =0.125, SF = 1.0738) ROWS  62500/13805.6585 CHIP 2, (1, PHASEDE-MO-TK, AD= 0.125, SF = 1.0738027,  GC = 1) ROWS  62500/14304.97612775 CHIP 3, (1,PHASEDE-MO-TK, AD = 0.125, SF = 1.0738054,  GC = 1) ROWS 62500/14804.295011 CHIP 4, (1, PHASEDE-MO-TK, AD = 0.125, SF =1.0738081,  GC = 1) ROWS  62500/15303.61514975 CHIP 5, (1,PHASEDE-MO-TK, AD = 0.125, SF = 1.0738108,  GC = 1) ROWS 62500/15802.936544 * * * CHIP 199, (1, PHASEDE-MO-TK, AD = 0.125, SF =1.0743346,  GC = 1) ROWS  62500/112695.034811 CHIP 200, (1,PHASEDE-MO-TK, AD = 0.125, SF = 1.0743373,  GC = 1) ROWS 62500/113194.60102775

Table 1 is an excerpt of a typical jobdeck of the commands issued to aMEBES III or 4500, illustrating the commands instructing it to write thefirst five and last two of 200 grating segments on a particularsubstrate. The MEBES jobdeck addresses each segment as CHIP followed bythe segment number. Referring to the jobdeck shown in Table 1,PHASEDE-MO-TK is the name arbitrarily given to the particular gratingpattern from which the grating segments written to the substrate aremodelled, AD is the address unit in microns prior to scaling and SF isthe scale factor.

The address unit is chosen at the design stage, as with the scalefactors, prior to the fabrication of the mask. AD is an integral divisorof the unscaled pitch of the grating pattern. A smaller value for ADresults in increased accuracy and resolution, whereas a larger valueresults in an improved write time. For an unscaled pitch of 1.0 micron,the address unit is typically either 0.1 micron or 0.125 micron.

The location of the center of the grating segment follows the ROWScommand, given as Y/X coordinates on an axis fixed relative to thesubstrate. As is well known in the art, a typical jobdeck provides allof these values established during the design phase for each of thegrating segments in the mask.

Execution of the jobdeck by the lithography tool is the next step, shownin FIG. 1 as 30. In this step, the grating segments that comprise thechirped fiber Bragg grating phase mask are written one-by-one onto thephotoresist coated substrate by exposing the photoresist to an imagewriting element such as an electron beam in a manner well known in theart. Control of the image writing element is carried out internally bythe MEBES tool based upon the commands in the jobdeck. As shown in steps30 a-30 e, the MEBES tool follows a specific set of procedures whencalled upon to write a grating segment to the substrate. First, as shownin 30 a, the MEBES tool retrieves the scale factor value for the nextsegment and calculates a new base writing unit for the mask by applyingthe scale factor to the address unit, 30 b.

In the next step, shown in 30 c, the MEBES tool performs are-registration and recalibration. This time consuming step is necessarywhenever the base writing unit is changed. As is known in the art, thecommand GC=1 shown in Table 1 (applied to CHIPs 2-200) reduces the otherrecalibrations undertaken by the MEBES tool to the minimum required toobtain properly scaled segments. As shown in 30 d, the MEBES tool writesthe grating pattern by exposing the photoresist according to the newbase writing unit at the axis location defined in the jobdeck.

After writing a segment to the substrate, the MEBES tool checks thejobdeck for the next segment. As shown at 30 e of FIG. 1, the MEBES toolwill repeat the steps shown at 30 a-30 d until the last of the gratingsegments has been written to the substrate and the array is complete.When the mask has been fully exposed, it is processed and used to form achirped fiber Bragg grating in an optical fiber in the conventionalmanner as described above, and shown in blocks 40 and 50.

Unfortunately, despite minimizing throughput overhead added due torecalibration of the MEBES tool to its minimum, the repetition of steps30 a-30 d still requires that for each successive segment the basewriting unit must be redefined to correspond to the new scale factor forthat segment. Because the pitch of each segment changes relative to theprevious segment in a chirped Bragg grating mask, the repeatedrecalibrations necessary at steps 30 b and 30 c can add significantthroughput overhead, especially for masks having an array with a largenumber of segments. This disadvantage can result in fabrication timesfor a typical 200 segment chirped Bragg grating phase mask to be over 8hours. From a commercial standpoint, this write time limits both thenumber of segments and the overall number of index modulations that canbe written onto the mask, and ultimately printed to the optical fiber.

Additionally, the technique is not sufficiently accurate for many fiberBragg grating applications. Stitching and pitch errors have beenobserved using the scale factor method that result in phase errors andunacceptable levels of a phenomenon known as group delay ripple (GDR) inthe chirped Bragg grating ultimately printed on the fiber using themask. GDR is the wavelength dependent deviation from the theoreticalgroup delay. Group delay is the time delay response curve across thereflected bandwidth of a chirped Bragg grating. GDR is normally reportedas the maximum peak to peak deviations from this curve measured inpicoseconds. A measurement of GDR indicates the degree to whichspatially induced wavelength dispersions are corrected by the fiberBragg grating. For example, errors typical in chirped fiber Bragggratings made with masks fabricated using the scale factor method on theMEBES III measure 60 picoseconds of GDR. The MEBES 4500, executing thesame jobdeck has produced masks measured at 30 picoseconds of GDR.Although the MEBES 4500 represents an improvement over the MEBES III,neither tool approaches the accuracy needed for critical applicationssuch as those in telecommunications, typically better than 10picoseconds of GDR.

It is known in the art to reduce GDR in a phase mask for a chirped fiberBragg grating by using a multipass writing strategy. For example, whenwriting a grating mask on an e-beam tool such as the MEBES or similarlithography tool using a multipass technique employing four passes, theintensity of the image writing element is reduced to ¼ of the intensityused to expose the substrate during a single pass. By shifting the errorboundaries, stitching error is reduced.

Although methods such as the multipass strategy achieve achieving asufficiently low GDR value in a mask produced by an e-beam or similarlithography tool, application of the technique is rendered commerciallyimpossible using the method of the prior art. At the typical ratediscussed above exceeding 8 hours per pass using the scale factormethod, the production time for a finished phase mask for a chirpedfiber Bragg grating applying the multipass technique can be measured indays. Although other lithography tools may have different write times,the effect of repeated recalibration of basic operational units issimilarly time-consuming.

Therefore a need exists for a method for writing chirped fiber Bragggrating phase masks using an e-beam or other lithography tool thatsignificantly reduces write time by avoiding repeated time-consumingrecalibration.

A further need exists for a method for writing chirped fiber Bragggrating phase masks presenting a reduced GDR in the fiber Bragg gratingsprinted therefrom.

SUMMARY OF THE INVENTION

In accordance with the invention, a phase mask for a chirped fiber Bragggrating defined by a series of scale factors is made without repeatedrecalibrations, thereby reducing throughput overhead. Instead, the scalefactors are converted into correction factors, or alpha corrections foreach segment. The position of each segment in the array that comprisesthe mask is adjusted to compensate for shifts in segment position due toapplication of the correction factor. This adjustment can be madefollowing the selection of a fixed “center point” from which thecorrection factor is measured in order to assure accurate stitching inthe mask pattern.

Thus, the phase mask for the chirped fiber Bragg grating is made in thecustomary manner by exposing a photoresist-coated substrate to anelectron beam or other lithographic instrument which writes the maskpattern into the substrate which is then developed and etched. Theresulting phase mask is used to write a chirped Bragg grating into anoptical fiber. However, by utilizing a correction factor at the segmentlevel to produce successive pitch changes in the mask, it is no longernecessary to redefine the base writing unit for each segment. Due todramatically reduced production times, the application of a multipasswrite strategy results in a highly accurate phase mask producedlithographically within an improved write time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the steps in making a chirped fiber Bragggrating using a phase mask produced on a lithography tool using themethod of the prior art.

FIG. 2 is a block diagram of the steps in making a chirped fiber Bragggrating having improved accuracy and reduced write-time.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, FIG. 2 is a block diagram illustrating thesteps in making a chirped fiber Bragg grating on lithography tool suchas the MEBES 4500 e-beam tool or similar lithographic tool using theimproved machine control method of the present invention. As in themethod of the prior art given in FIG. 1, a photoresist coated substrateis loaded onto the stage of the lithography tool, and an appropriategrating pattern and scale factors are chosen as shown in FIG. 2 at 110and 120 respectively.

However, instead of repeatedly applying scale factors to the addressunit, the scale factors used in the method of the prior art areconverted to correction factors applied directly to each segment, knownfor example, to a MEBES tool as “alpha” corrections. In this case X-axisalpha corrections are used because the pitch of each segment isdependent upon its X-axis dimension. A process of converting the scalefactors is set forth below. Although the steps provided are directedspecifically toward the syntax required for execution on a MEBES tool,it will be understood by one skilled in the art that the process isadaptable to any e-beam or lithography tool having similar capability tothose of the MEBES tool. Furthermore, it will be understood that thesteps and computations set forth below and in the drawings are merelyillustrative of the preferred embodiment of the invention and are notprovided for limiting same.

Block 130 of FIG. 2 illustrates two possible steps to make the necessaryconversion from scale factor to alpha correction. First, as shown inblock 130 a, a fixed reference scale factor and address unit are chosen,then as shown in block 130 b, the scale factors can be converted intoequivalent segment level correction factors (alpha corrections).

In applying step 130 a, the scale factor and address unit of any onesegment in the grating can provide the needed reference, however it ismore convenient to select a hypothetical segment that is just outsidethe grating to minimize the absolute magnitude of the alpha correctionsnecessary, and also to prevent a sign change in the value of thecorrections from one end of the array of grating segments to the other.A reference scale factor may be extrapolated, for example, by thefollowing equation:

SF _(ref) =SF _(last)+(SF _(last) −SF _(last−1))

Where SF_(ref) is the scale factor of a hypothetical segment outside themask, SF_(last) is the last segment in the grating, and SF_(last−1) isthe penultimate segment.

Applying the equation numerically, for example, to obtain SF_(ref) forthe grating defined by the jobdeck example given in Table 1, where thevalue for SF_(last) (1.074337300) is found in CHIP 200, and the valuefor SF_(last−1) (1.074334600) is given in CHIP 199, the equation yieldsan SF_(ref) of 1.074340000

Having established a reference scale factor, conversion of scale factorvalues into alpha correction values as called for in step 130 b iscarried out by the following equation:

XA _(n)=(SF _(n) −SF _(ref))SF _(ref)

Where SF_(n) is the scale factor of the segment to be converted,SF_(ref) is the scale factor of the reference segment and XA_(n), is thealpha correction in the X-axis dimension of the segment. Following thenumeric example given in Table 1, the scale factor for segment 1, givenin the jobdeck as CHIP 1, at SF, is 1.0738. Applying the above equationto that value, and incorporating the value for SF_(ref) (1.074340000)derived in the previous step 130 a, the value for XA_(n) for segment 1is −0.000502634. This conversion must be undertaken for each of thesegments in the mask prior to submitting the job to the MEBES tool.Therefore, step 130 may be repeated as many times as necessary.

Application of an alpha correction by the MEBES tool results in aspecific amount of expansion or contraction of the segment pattern inthe X-axis equivalent to the results accomplished by applying a scalefactor to the base writing unit according to the method of the priorart. An alpha correction can be used in place of a scale factor becausethe alpha command effectively “scales” each segment directly. Therefore,the grating pattern ultimately written to the substrate by the MEBEStool should theoretically be the same using either method. However,because the application of an alpha correction does not involverecalibration of the base writing unit, as discussed below, an expectedshift in the X-position of the segments is created when an X-Axis alphacorrection is applied to the first segment, and accumulates for eachsubsequent alpha-corrected segment.

The next step, indicated by block 140 of FIG. 2, is to anticipate andcorrect for the expected shift in the X-position of the segments due tothe application of the alpha correction. The magnitude of this shiftdepends upon the distance of the desired location from a fixed referencepoint. All shifts using alpha corrections are calculated from the samereference point and all coordinates on the mask will expand or contractaround this location for any given alpha value based on whether the signis positive or negative. For a Mebes 4500 this reference point is acalibration grid mounted on the stage and fixed relative to thesubstrate. This grid is composed of a 2 mm diameter silicon wafer whichhas been etched into a 13×13 lattice with bars approximately 38 micronswide and spaced approximately 160 microns apart. The stage coordinatesof the intersection of one vertical and one horizontal bar of this gridare stored in a parameter file and used for all image writing elementcalibrations. Repeated scanning over a long period eventually wears thesilicon bars until this location can no longer be used and a new gridintersection must be picked. The stage location of this new intersectionis recorded by the MEBES tool. The X value, known on the Mebes 4500 asXFID, is used as the fixed reference point for the calculations tocorrect X-axis shift. Because XFID, or X-axis mask coordinate can varyby up to 2 mm over time due to the periodic exhaustion of referencepoints, it is necessary to use the most current number in thecalculations. It is ultimately possible for this number to vary by asmuch as 13 mm since there is a complete second 2 mm grid on the stagefor use when the first one is completely worn out. For example an X-axismask coordinate on one of these grids is 41,157. The chosen coordinateis always stored by the MEBES tool with 0-digit accuracy but allcalculations using it are performed to 9-digit accuracy.

The second step, shown in block 140 b in FIG. 2 is to calculate anoffset X-location, X_(na), on the substrate that, upon application ofthe alpha correction, will result in the correct placement of eachsegment relative to the others in the grating given by the followingequation:

X _(na)=(X _(n) −X _(c))/(1+A _(n))+X _(c)

where X_(n) is the correct X-location on the grid for segment n toachieve the proper stitching, A_(n) is the alpha correction applied tosegment n, and X_(c) is the fixed reference or X-axis mask coordinateselected in the previous step, shown in block 140 a. X_(n) can bederived from the desired grating established in step 120, and is givenfor segment 1 numerically, for example, in the sample jobdeck of Table 1as 13805.6585. Thus, given an X_(c) fixed numerically at 41,157 in Step140 a, the offset X-location for segment 1 in view of its alphacorrection value A_(n) of 0.000502634 is 13791.903872271. In order toensure proper stitching of each segment, the offset X-location must becalculated for every segment.

The next step, shown in block 150 of FIG. 2, is to establish an addressunit, shown in block 150 a and to provide the alpha corrections andcorresponding offset X-locations to the lithography tool. As noted aboveand shown in block 150 b, a jobdeck is used for this purpose on a MEBEStool. The jobdeck of Table 2 mirrors that of Table 1, except that thescale factors are replaced by an X-axis alpha correction preceded bycommand XA. Therefore, to obtain the same finished mask as called for bythe commands in the jobdeck of Table 1, the X-values following the ROWScommand in Table 2 are the offset X-values calculated in step 140.

Furthermore, the value for AD in Table 2 may be established by applyingthe reference scale factor SF_(ref) to the AD of Table 1, thereby fixingthe base writing unit to a single value for the entire job. Numerically,the AD of Table 1 (0.125) is multiplied by SF_(ref) (1.07434) yieldingan AD for Table 2 of 1.342925.

Matching jobdeck alpha example CHIP 1, (1, PHASEDE-MO-TK, AD =0.1342925, XA =  −0.000502634) ROWS  62500/13791.903872271 CHIP 2, (1,PHASEDE-MO-TK, AD = 0.1342925, XA =  −0.000500121) ROWS 62500/14291.540147102 CHIP 3, (1, PHASEDE-MO-TK, AD = 0.1342925, XA = −0.000497607) ROWS  62500/14791.175192014 CHIP 4, (1, PHASEDE-MO-TK, AD= 0.1342925, XA =  −0.000495094) ROWS  62500/15290.80895376 CHIP 5, (1,PHASEDE-MO-TK, AD = 0.1342925, XA =  −0.000492581) ROWS 62500/15790.441459227 * * * CHIP 199, (1, PHASEDE-MO-TK, AD =0.1342925, XA =  −0.000005026) ROWS  62500/112695.329678025 CHIP 200,(1, PHASEDE-MO-TK, AD = 0.1342925, XA =  −0.000002513) ROWS 62500/113194.749716305

During execution of the jobdeck by the MEBES tool, shown in block 160 ofTable 2, the MEBES tool retrieves data for the first segment, CHIP 1 asshown in block 160 a. The MEBES tool then calculates the base writingunit from the address unit, as shown in block 160 b, and calibratesitself based on the address unit, as shown in block 160 c in the samemanner as steps 30 b and 30 c of FIG. 1. The MEBES next applies thealpha correction to the grating pattern and the offset X-position forthe first segment, as shown in block 160 d, and writes the segment tothe substrate as shown in block 160 e. Because no calibration of theMEBES tool takes place during the application of the alpha correction,the GC=1 command is superfluous and therefore omitted from the jobdeckof Table 2.

In the next step, shown as block 160 f of FIG. 2, the MEBES tool checkswhether the last segment has been written. If the last segment has notbeen written, the MEBES tool retrieves the data for the next segmentfrom the jobdeck, as shown in block 160 g. Because the address unit isthe same for every segment, and because the application of alphacorrections by the MEBES tool does not require recalculation of the basewriting unit or recalibration steps, the MEBES tool merely applies thenext alpha correction to the grating pattern at the corresponding offsetX-position and writes the segment to the substrate as shown in blocks160 d and 160 e. This process is repeated until the last segment hasbeen written to the substrate. When the last segment has been writtenthe mask may be re-exposed according to the multipass techniquedescribed above and shown in block 160 h. If an additional exposure isrequired, the lithography tool returns to the step shown in block 160 a.When the last pass is completed the substrate is ready for processingand printing, as known in the prior art and shown in blocks 170 and 180.

As can be seen from the process shown in FIG. 2, when compared to theprocess conventionally known in the art, the method of the presentinvention avoids the repeated recalculation and recalibration due to thebase writing unit called for in the conventional method. The result isan improvement in write time on the MEBES 4500 from approximately 2.5minutes per grating segment using the conventional method toapproximately 3 seconds per grating segment to execute the equivalentjobdeck. A similar improvement can be expected from any lithography toolcarrying out similar sets of commands.

As noted above, significant additional pitch and stitching accuracy, aswell as an increase in the number of pitch changes across a finishedgrating mask can be accomplished by incorporating known techniques suchas multipass averaging with the method of the present invention. Becauseof the significant improvement in write time observed using alphacorrections, it is practical, using this method, to make multiple passesover a single mask in the same or less time than required to write asingle pass of a mask using scale factors.

The invention is not limited to the embodiments described above, but allchanges and modification thereof not constituting departure from thespirit and scope of the invention are intended to be included.

What is claimed is:
 1. A method for fabricating with a lithography tool a phase mask for a chirped fiber Bragg grating having a plurality of progressively chirped grating segments stitched together comprising the steps of: providing a substrate having a planar surface coated with photoresist; establishing a fixed reference on said substrate; establishing a set of scale factors corresponding to said progressive chirp of said grating segments; calculating a reference scale factor for a hypothetical segment outside of said plurality of progressively chirped grating segments; and calculating a correction factor for each segment using said hypothetical segment as a reference; dividing said substrate into an array of said plurality of grating segments relative to said fixed reference and according to said correction factor whereby each grating segment is stitched to the segments adjacent thereto; exposing said photoresist to an image writing element of said lithography tool according to said array; developing said photoresist; and etching said substrate to produce a phase mask having a plurality of progressively chirped surface relief grating segments.
 2. The method of claim 1 wherein the step of exposing said photoresist to said image writing element of said lithography tool according to said array is repeated to perform multipass averaging prior to the developing of said photoresist.
 3. The method of claim 1 wherein said array of said plurality of grating segments are comprised of grating segments derived from a common grating pattern.
 4. A method for fabricating with a lithography tool a phase mask for a chirped fiber Bragg grating having a plurality of segments each defined by a grating pattern, a scale factor and a position on an axis comprising the steps of: providing a substrate having a planar surface coated with photoresist; calculating a reference scale factor for a hypothetical segment outside of said plurality segments; calculating correction factors from said scale factors relative to said hypothetical segment; calculating the axis shift of each segment from the position on said axis corresponding to said correction factor; applying sequentially said correction factor to said grating pattern to divide said substrate into said plurality of segments; exposing sequentially said photoresist to an image writing element of said lithography tool according to each of said plurality of segments at a position on said substrate corresponding to said axis shift; developing said photoresist; and etching said substrate.
 5. The method of claim 4 wherein the step of exposing sequentially said photoresist to said image writing element of said lithography tool according to each of said plurality of segments at a position on said substrate corresponding to said axis shift is repeated to perform multipass averaging prior to the developing of said photoresist. 